The invention relates generally to detecting and tracking aircraft using coded reply signals transmitted from aircraft transponders. More specifically, the invention relates to such detecting and tracking under conditions in which multiple and multipath reply signals are received during a receive interval following an interrogation.
Air traffic control and safety are ongoing concerns in commercial and military aviation. Particularly significant concerns are traffic alert and collision avoidance between aircraft either in route between or in the vicinity of landing fields. Ever increasing air traffic demands have resulted in governmental regulations that require commercial carriers to equip planes with active interrogation systems that can determine the presence and threat of nearby aircraft called targets. The particular system mandated by the government depends on the aircraft size. Large commercial aircraft that carry over 30 passengers are being equipped with an active traffic and collision avoidance system (TCAS II) that not only detects and displays nearby aircraft, but also alerts the crew as to impending collisions, and also provides resolution advisories such as audible instructions to the pilot to pull up or down, maintain level or climb rate and so forth. This system, however, is very complex and expensive and therefore has not been mandated for smaller aircraft.
For aircraft that carry up to 30 passengers, governmental regulations require such aircraft be equipped with an active interrogation system (TCAS I) that detects nearby aircraft, determines and displays range, bearing and altitude of such aircraft relative to the interrogating plane, and tracks such aircraft within a prescribed range and issues an audible alert to the crew as to impending collisions. Although the operational performance of the TCAS I system appears less complex than TCAS II, numerous problems arise that make a cost effective system difficult to realize.
The Federal Aviation Administration (FAA) specifies that the TCAS I active interrogation systems use air traffic Control radar beacon system (ATCRBS) signals. These ATCRBS interrogation signals are high frequency pulse amplitude modulated signals at 1030 Megahertz. The reply signals are also pulse amplitude modulated but at a carrier frequency of 1090 Megahertz. In TCAS I, the reply and interrogation signals are transmitted from an interrogation aircraft to other aircraft in the vicinity thereof, and these other aircraft respond to the interrogations via a transponder located on the aircraft.
The interrogation and reply signal waveforms are specified by the FAA. The information contained in the reply signal depends on the type of interrogation (e.g. Mode A, Mode C) and the transponder equipment that the interrogated aircraft has available for responding. For TCAS I, the interrogation mode is Mode C, and the Mode C reply signal from the aircraft transponder consists primarily of encoded altitude data. The altitude data is encoded using binary logic states or bits arranged in four digit octal codes (i.e. each octal altitude code has twelve data bits with each octal digit defined by three data bits). The reply signal data bits are transmitted within a pair of framing pulses called bracket pulses that indicate (for purposes of TCAS I) the beginning and end of an altitude code reply signal from a particular aircraft responding to an interrogation.
A TCAS I system is specified based on the use of these ATCRBS Mode C reply signal waveforms. Thus, an interrogating aircraft may transmit an interrogation signal at 1030 MHz, and then will "listen" for Mode C reply signals from all aircraft capable of responding by transmitting the bracket pulses and altitude encoded data pulses. Some aircraft are not equipped to reply with altitude data (non-altitude reporting, or NAR) and hence only transmit the bracket pulses. Under TCAS I, aircraft within a range of about 34 nautical miles will reply to a Mode C interrogation.
A significant problem that is present during the time that an interrogating aircraft is "listening" for replies, is the occurrence of multipath replies. When a target (responding) aircraft transmits a reply signal, the signal is transmitted substantially omnidirectionally. In addition to the direct transmission path between the target aircraft and the interrogating aircraft, there can be indirect paths that the reply signal can follow back to the interrogating aircraft. For example, the reply signal may be reflected from the Earth's surface (ground or water), or mountains, large buildings and so on. This results in the interrogating aircraft apparently receiving more than one reply signal from the same target. These additional replies must be adequately identified so that they are not misinterpreted as new targets to be tracked, or misinterpreted to provide inaccurate range, bearing and altitude information of the real or true target.
The presence of redundant and false reply signals also will tend to slow down the signal processing used to develop the data needed for target acquisition and tracking, because each reply signal must be analyzed to determine if it represents data from a target that could be a threat to the interrogating aircraft.
In addition to multipath replies, interrogating aircraft with TCAS I systems use an interrogation sequence called a "whisper/shout" sequence. The basic concept for using whisper/shout is to try to reduce the number of aircraft responding at the same time to an interrogation. Accordingly, the interrogation sequence is controlled so that close-in targets will respond first, then farther targets will respond. This is basically accomplished by limiting the signal strength of the initial interrogations so that only nearby aircraft will "hear" the interrogation and respond thereto. However, a target may still reply more than once an interrogation sequence.
Although the whisper/shout sequence can help reduce the number of targets responding at the same time, under the TCAS I specification the number of reply signals received can still be excessive, particularly in high traffic areas.
The objective exists, therefore, for a traffic alert and collision avoidance system that can identify and reject multiple reply signals, particularly reply signals received from indirect multipath reception.